1. Field of the Invention
The present invention relates to a structure for protecting a rod integrator and a projection display apparatus using the structure especially used as a digital cinema projector.
2. Description of the Related Art
With the reduction in size and improvement in performance of projection display apparatuses, which has been brought about through a rapid advance in liquid crystal technologies and DLP (Digital Light Processing; trademark) technologies, projection display apparatuses for projecting an image have been used in wider fields. The projection display apparatus is drawing attention as a front projector, which is a large projection display apparatus that is replacing conventional home-use television sets. The projection display apparatus has also been used for showing movies, as a digital cinema (DLP cinema) projector, because of the high resolution panel that enables the apparatus to project high-resolution images at full size.
Conventionally, a movie is displayed by projecting images stored in a video film onto a very large screen, for example, in a theater. Digital cinema is common to the conventional cinema in that images are projected and displayed on a very large screen. However, the digital cinema is different from the conventional cinema in that digital data are used for projection, instead of a film, and are projected onto a screen by a projection display apparatus, or by a digital cinema projector. The digital cinema largely differs from a front projector, which is used, for example, in a classroom or a conference room, in size, brightness, and reliability.
For example, a front projector typically employs an ultra-high pressure mercury lamp of approximately 300 W at the highest as a light source, whereas, a digital cinema projector requires a xenon lamp of 2 kW to 6 kW. Therefore, the amount of heat that is generated by a lamp and the size of a lamp, as well as danger, are incomparably larger than that of a front projector, and the entire size and the total weight of the apparatus also incomparably larger than that of a front projector.
A beam of light that is emitted from a xenon lamp based on an arc discharge usually has a circular cross section. However, the video display surface of a display device for generating images has a rectangular shape having an aspect ratio of, for example, 4:3, or 16:9, similar to images that are projected onto a screen. Although it is possible to cover the entire video display surface in the shape of a rectangle with a beam of light having the circular cross section, the circular cross section needs to be larger than a circle that circumscribes the rectangular video display surface. Since the area of the beam that is actually incident on the video display surface is smaller than the entire area of the beam, a large portion of the light is lost. Another disadvantage of this method is that the video display surface is brighter in its central region and darker in its peripheral region, and therefore, uniform brightness of a screen cannot be obtained. To cope with this problem, a special illuminating optical system using an element for uniformizing illumination, such as an integrator lens, which is a kind of fly-eye lens, a tubular light pipe, or a light tunnel, is used. The special illuminating optical system increases efficiency by converting a circular beam of light into a rectangular beam of light, which is similar to the shape of the video display surface, and also increases uniformity of brightness by overlapping light beams. Therefore, an integrator lens, a light pipe, or a light tunnel generally has a rectangular cross section which is similar to the rectangular video display surface of a display device. Digital cinema projectors often employ a xenon lamp as a high-output lamp and a display device comprising three DMDs (Digital Micromirror Devices). In such a digital cinema projector, a special illuminating optical system that employs a light tunnel or a light pipe (hereinafter referred to as a “rod integrator”) is often used in combination with the projector in order to improve optical performance. In this arrangement, a beam of light having a circular cross section that is emitted from a lamp converges onto a rod integrator having a rectangular cross section by means of a lens or the like.
However, a lamp actually has a light-emitting point in a finite size. The point on which light converges also has a finite size. Further, a beam of light contains components that undesirably diverge from the main beam of the light. Therefore, it is impossible to make the entire beam of light converge onto the rod integrator within the rectangular cross section thereof. The undesirably diverging light is cut off by the opening of an element that constitutes a support mechanism which holds optical components, such as a rod integrator, and is also cut off by an incident surface of the rod integrator. The light that is radiated on the component of the support mechanism heats the component, causing an early deterioration of the component and making it difficult to keep the component reliable. If the converging light is emitted from a high-output lamp, then the metal component will soon be deteriorated unless it is cooled. The undesirably diverging light poses a large problem if it is emitted from a high-output lamp for use in a digital cinema projector, though it is not problematic if the light is emitted from a low-output lamp up to 300 W.
Japanese Patent Laid-Open Publication No. 2002-318423 discloses a video display apparatus having light shield plates to prevent undesirable illuminating light from entering optical components, and to prevent an excessive rise in the temperature of the optical components. The light shield plates are positioned in front of a color filter and a rod integrator that are disposed along the path of illuminating light. A heat radiator, which is integrated with each light shield plate, extends out of a mirror chamber to radiate undesirable heat energy that is inside the video display apparatus to the outside.
According to related art other than the art disclosed in the above-mentioned patent document, in order to cool optical components, cooling fins are provided on rod integrator holder 141 which constitutes support mechanism 120 for supporting an illuminating optical system. Rod integrator holder 141 is cooled by cooling air that is supplied to the cooling fins by means of cooling fan 161, as shown in FIG. 1. The illuminating optical system has a light source lamp that is housed in lamp housing 111, cold mirror 112 for reflecting light that is emitted from the light source lamp and for separating an infrared radiation from the light, a rod integrator, not shown, for receiving the light that is reflected by cold mirror 112, support mechanism 120 for supporting illuminating optical system, and display device unit 170 having an image display unit. Support mechanism 120 includes rod integrator holder 141 that houses and supports the rod integrator.
If a light source lamp of a high-output type is used, then the influence of the rod integrator on the incident surface is one of the largest concerns. Therefore, rod integrator holder 141 that houses the rod integrator therein has an opening whose shape is similar to the cross section of the incident surface of the rod integrator, and has cooling fins with a high cooling capability on its outer surface.
In recent years, the output of a lamp has been increasing to 6 kW or 7 kW in order to meet the requirements for higher brightness of an image. However, increased output has made it difficult to maintain optical component reliability for a long period of time as a result of the increased influence of undesirable light on optical components. In addition, a lamp used for a digital cinema projector is often replaced with another one having a different output, and other components of the digital cinema projector are used as they are because of, for example, the high cost of the projector. Specifically, only the lamp of a digital cinema projector is replaced with a lamp having a different output in order to use the digital cinema projector as a projector having a different luminance level, or in order to adjust the luminance level depending on the site where the digital cinema projector is installed, based on the consideration that a digital cinema projector is intended for a special purpose, i.e., digital cinema projection. In this example, it is necessary to prevent undesirable light and to keep the illuminating optical system cool for various kinds of lamps that may be used and that may have different output levels. Under these circumstances, a related art projection display apparatus are no longer capable of keeping components reliable for a long period of time.